This invention relates generally to apparatus and methods for controlling the frequency of light output from an optical signal source. This invention is particularly related to apparatus and methods for controlling the frequency of optical signals output from coherent light sources used in fiber optic rotation sensors.
Stability in the frequencies input to optical fibers is a practical necessity in the development and implementation of sensing systems using optical fibers. Optical sensing systems may use semiconductor diode lasers or superluminescent diodes as light sources. A high-precision fiber optic rotation sensor requires a stable light source because the scale factor of the sensor depends upon the source wavelength. For example, a navigation grade rotation sensor requires wavelength stability of about one part in 10.sup.6. A wideband source such as a superluminescent diode (SLD) or a narrower source such as a single or multimode laser diode needs frequency stabilization in order to be suitable as an optical source for a Sagnac ring fiber optic rotation sensor.
The SLD provides a spectral line width sufficient to overcome unwanted phase errors due to coherent backscatter and the Kerr effect. The fractional line width should be between 10 and 1000 parts per million (ppm). The frequency stability of the centroid of the source spectral distribution should be several ppm to meet scale factor stability and linearity requirements. Therefore, source width should be minimized within the constraints of coherent backscatter and Kerr effect errors to enhance scale factor linearity. The fractional line width should approach the lower portion of the 10 to 1000 ppm range to minimize unwanted errors in scale factor due to changes in the source spectral distribution over time.
The wavelength of the light emitted from a laser diode varies as a function of the operating temperature and the injection current applied. Effective use of a superluminescent diode as a light source in an optical rotation sensor requires an output of known wavelength. In fiber optic rotation sensing applications, the frequency stability should be about .DELTA.f/.sub.f =10.sup.6, and the light source should be held at a constant temperature.
U.S. Pat. No. 4,842,358 which issued to the present inventor and which is assigned to Litton Systems, Inc., the assignee of the present invention, discloses optical signal source stabilization using an interferometer to form to optical beams. The disclosure of U.S. Pat. No. 4,842,358 is hereby incorporated by reference into the present disclosure. U.S. Pat. No. 4,842,358 discloses a pair of birefringent crystals placed in the optical path of a light beam output from an optical signal source. The crystals have polarization dependent refractive indices and produce a first beam having an intensity I.sub.o (1+cos .phi.) and a second beam having an intensity I.sub.o (1-cos .phi.). At the desired source frequency the two intensities are equal. The difference of the two intensities is used to form an error signal that is used to servo the source drive current to produce a signal having a frequency that minimizes the error signal. Instead of using two crystals to form an interferometer, a fiber optic Mach-Zehnder interferometer may be used to form the two beams that are processed to form the error signal.